The invention relates to an inkjet printing system and an inkjet printing process performable with this system.
Inkjet printing systems are used in various ways in the printing industry. In these printing systems, tiny ink droplets are produced by computer-controlled nozzles, and applied to a printing material. In particular, systems wherein the nozzles are fitted with piezoelectric elements have been tried and tested. In these systems, all the piezoelectric elements are supplied with electrical signals by a common pulse generator, the individual piezoelectric elements, respectively, being switched on as a function of the imaging information to the pulse generator. Thus, all the nozzles that are not switched on do not output any ink droplets, and all the nozzles which are switched on output ink droplets of the same size. The simultaneous production of ink droplets of different sizes is consequently not possible with the heretofore known system of this general type, i.e., different gray values cannot be printed at the same time with the heretofore known system.
In order to display different gray values, in the case of the heretofore known system, it is necessary for the nozzles to be activated repeatedly at short time intervals or for the printing material to be guided repeatedly past the same nozzle arrangement or for a plurality of nozzle arrangements to be positioned in tandem, i.e., behind one another.
The heretofore known inkjet printing system thus has the disadvantage that always only one droplet size and, consequently, also only one gray value can be produced simultaneously by the nozzles. In other words, although it is possible to control all the nozzles individually and it is also possible to vary the control signal and, therefore, the droplet size, a limitation or restriction exists in that all the nozzles controlled at the same time are controlled with the identical signal, and therefore identical droplet sizes are also produced. At another time, a different droplet size can be produced, but this is in turn identical for all the nozzles controlled at this different time.
U.S. Pat. No. 5,265,315 discloses a method of producing a thin-layer inkjet head. By the heretofore known method, an electrode arrangement is produced which, as can be seen from the drawing, appears to have a plurality of electrodes for each nozzle. The question, however, as to whether these are definitely separate electrodes or whether the electrodes are interconnected is left open in this document. Also left open is the question as to which functions are given to this specific electrode arrangement and how the electrodes are controlled in detail.
It is accordingly an object of the invention to provide an inkjet printing system and an inkjet printing process which has greater flexibility with respect to the size of the ink droplets produced, respectively, than heretofore possible with conventional inkjet printing systems.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, an inkjet printing system comprising an arrangement of nozzles, respectively, having a nozzle chamber formed with a nozzle opening and provided with a respective piezoelectric element, and a control device for controlling the piezoelectric elements, the control device having at least two signal paths switchable-on individually for each of the nozzles.
In accordance with another feature of the invention, each of the piezoelectric elements has at least two control electrodes individually controllable by the control device.
In accordance with a further feature of the invention, the control electrodes of the same piezoelectric element are of different construction, so that ink droplets of different sizes are producible with an identical signal.
In accordance with an added feature of the invention, the control electrodes of the same piezoelectric element are arranged in a non-equivalent manner, so that ink droplets of different sizes are producible with an identical signal.
In accordance with an additional feature of the invention, the control device has a plurality of signal sources corresponding to the number of signal paths for each nozzle, each of the signal sources serving for producing different signals.
In accordance with yet another feature of the invention, the control device has at least one signal source and at least one modifier circuit for modifying the signals provided by the signal source.
In accordance with yet a further feature of the invention, the inkjet printing system further comprises a modifier circuit assigned to at least one signal path, respectively, for each of the nozzles.
In accordance with yet an added feature of the invention, the modifier circuit is constructed as an RC element.
In accordance with yet an additional feature of the invention, the control device has a switch matrix for switching-on the signal paths to the piezoelectric elements.
In accordance with another aspect of the invention, there is provided an inkjet printing process for printing with ink droplets on a printing material, which comprises providing an arrangement of nozzles for producing the ink droplets, providing a control device for controlling piezoelectric elements assigned to the nozzles, and controlling with the control device a respective piezoelectric element in order to produce an ink droplet, by switching-on at least one of at least two signal paths provided individually for controlling the respective piezoelectric element.
In accordance with another mode, the process of the invention further comprises controlling with the control device at least one of at least two electrodes of the piezoelectric element based upon the respectively desired size of the ink droplet.
In accordance with a concomitant mode, the process of the invention further comprises controlling the piezoelectic element with the control device by a signal which is prescribed based upon the respectively desired size of the ink droplet.
The inkjet printing system according to the invention comprises an arrangement of nozzles each having a nozzle chamber formed with a nozzle opening and provided with a piezoelectric element. Furthermore, the inkjet printing system according to the invention has a control or driving device having at least two signal paths which are switchable-on individually for each nozzle.
The inkjet printing system according to the invention offers the advantage that, in relation to nozzles controlled at the same time, ink droplets can be produced in at least two different sizes, depending upon which signal path, respectively, of the control device is switched.
In a preferred embodiment, each piezoelectric element has at least two control electrodes, which are driven individually by the control or drive device. This provides the advantage that the production of different droplet sizes can be implemented or realized in many different ways.
One possible realization calls for the control electrodes of the same piezoelectric element to be constructed differently, so that ink droplets of different sizes can be produced with an identical signal. This has the advantage that only one signal source is required in order to drive the inkjet printing system.
Furthermore, the control electrodes of the same piezoelectric element can be arranged in a non-equivalent manner. This measure also permits the production of ink droplets of different sizes with one and the same signal.
In a second embodiment of the inkjet printing system according to the invention, it is sufficient for each piezoelectric element to have a single control electrode. In this embodiment, the control device has a plurality of signal sources corresponding to the plurality of signal paths for each piezoelectric element, each signal source producing different signals. This embodiment offers the advantage that conventional nozzles with one control electrode for each piezoelectric element can be used.
In a third embodiment of the inkjet printing system according to the invention, the control device has at least one signal source and at least one modifier circuit for modifying the signals provided by the signal source. This embodiment has the advantage that it needs only one signal source. Nevertheless, in the case of this embodiment, analogous with the second embodiment, nozzles with a single control electrode can be used.
The third embodiment can, in particular, also be constructed so that, respectively, one modifier circuit is assigned to at least one signal path per nozzle.
The modification circuit is preferably realized or implemented as an RC element or component.
In all of the embodiments of the inkjet printing system according to the invention, the control device can have a switch matrix for switching on the signal paths to the piezoelectric elements. The use of a switch matrix has the advantage that it can be implemented compactly and cost-effectively.
The inkjet printing process according to the invention provides for producing, by an arrangement of nozzles, ink droplets for printing a printing material, and controlling, by a control device, the piezoelectric elements assigned to the nozzles. In this regard, the control device controls the appropriate piezoelectric element, respectively, in order to produce an ink droplet, by switching on at least one of at least two signal paths provided individually for controlling this piezoelectric element.
In a preferred modification, the process includes controlling, by the control device, at least one of at least two electrodes of the piezoelectric element, based upon the respectively desired size of the ink droplet.
In a further modification, the process includes controlling the piezoelectric element, by the control device, with a signal which is prescribed or predefined based upon the respectively desired size of the ink droplet.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an inkjet printing system and an inkjet printing process, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.